Until recently, cooling for computers, particularly laptop computers has been primarily accomplished by the use of conduction, natural convection, forced air convection, e.g. through surface dissipation of heat to the environment through heat sinks or enclosure surfaces, through the use of fans, and in many cases, the attachment of fans and heat sinks directly to heat generating components such as processors and the like. However, as processing power and circuit density increase on portable computer systems, cooling requirements may not adequately be met by conventional methods and structures. In addition, a primary heat dissipating surface for a laptop computer, e.g. the underside of the base, is often in direct contact with a work surface and thus is prevented from achieving maximum dissipation or even a predictable degree of dissipation.
In addition, to the extent that forced air systems continue to be used and improved, noise associated not only with fan mechanisms, but with the volume of moving air required to cool advanced processors becomes a problem. Such noise may be acceptable in the mobile computing environment, but in a relatively quiet environment such as an office where, for example, a portable computer may be attached to a docking station for use within the office environment on a desktop, noise levels associated with forced air convection from fans and the like may become unacceptable.
In many systems where heat removal is an important consideration, liquid based cooling systems may be used particularly on major heat generating components such as the display, and the like. In U.S. Pat. No. 5,791,770 issued on Aug. 11, 1998 to Hoyt, et al a cooling device is described for an LCD display the device having a sealed container into which the light source is immersed along with thermally conductive liquid coolant. Drawbacks of such a cooling device might include the lack of ability of the cooling device to address cooling needs of the processor and other heat generating components within the system. To address system wide cooling, U.S. Pat. No. 6,313,990 B1 issued on Nov. 6, 2001 to Cheon, describes a liquid cooling system wherein a pump is used to circulate liquid coolant from a finned reservoir throughout the entire electronic device. Heat absorbed by the liquid coolant is dissipated when a fan circulates ambient air across the reservoir fins. However, as described, the use of a fan is noisy and undesirable in quiet environments such as offices. While typical laptop environments are generally less sensitive to noise; when a laptop is used in an office environment, fan noise would be considered undesirable.
Some laptops or portable computer systems have been equipped with liquid cooling systems. The portable computer described in U.S. Pat. No. 5,634,351 issued on Jun. 3, 1997 to Larson, et al (hereinafter “Larson”), for example, has a two-phase cooling system residing in a base or lid of a portable computer near the circuitry to be cooled. The system of Larson uses passive wicking to circulate liquid throughout the system conserving battery power. Other liquid based cooling systems for portable computers such as that described in U.S. Pat. No. 5,757,615 issued on May 26, 1998 to Donahoe et al., use motor driven pumps to circulate coolant through the system to heat exchangers. Accordingly, it can be appreciated that liquid cooling is an effective means of circulating heat away from heat generating components by absorbing heat into the relatively high heat capacity liquid and circulating the liquid either passively or by pump to a heat exchanger however a complete liquid cooling system within a portable computer adds undesirable weight and bulkiness.
Air convection may also be effective in removing heat and may be simpler in that air may be exhausted into the atmosphere, though it should be noted that air lacks the heat absorbing capacity of many cooling fluids. Fans may further be used to improve the rate of air exchange in a space and control where exhaust air is directed. While fans may be effective in moving heated air from an enclosure or away from the surface of a device or heat sink, other non-liquid, conduction based devices may be used to draw heat from a source where fans are impractical or undesirable for whatever reason.
The simplest mechanical conduction device well known in the art is the heat sink. The heat sink takes advantage of materials with high thermal conduction properties coupled with increased surface area within the area to be cooled. The heat sink is often used in connection with other devices such as fans and the like to remove the heated air which dissipates from the surfaces of the heat sink from an enclosure. As real estate inside portable devices shrink, so does the effectiveness of heat sinks since a heat sink's effectiveness is proportional to the amount of surface area associated with its dissipating surfaces. Another device often used in connection with a heat sink is a heat pipe.
A heat pipe in its simplest form is a thermally conductive material formed into a bar or pipe and often is terminated with a heat sink. Heat pipes may also be more complicated, sealed and filled with liquid or the like. The operation of the heat pipe is to draw heat along its thermally conductive length to a sink area where heat can be safely dissipated such as outside an enclosure or the like. In U.S. Pat. No. 6,459,575 B1 issued on Oct. 1, 2002 to Esterberg, for example, a cooling module is described wherein a heat pipe associated with the cooling module is inserted into a heat sink associated with a device such as a portable computer or the like. Heat is drawn from the heat sink along the heat pipe to the cooling module where the heat is exhausted into the atmosphere using a fan. In other systems, such as that described in U.S. Pat. No. 6,181,553 B1 issued on Jan. 30, 2001 to Cipolla et al, heat may be transferred from a portable computer to a docking station through the use of a heat pipe and heat sinks located in the docking station. In a similar configuration, as described in U.S. Pat. No. 6,434,001 B1 issued on Aug. 13, 2002 to Bhatia, heat pipes connected to heat sinks and associated with a docking station may be removably coupled to heat transfer elements of a portable device. Still further, for example as described in U.S. Pat. No. 6,453,378 B1 issued on Sep. 17, 2002 to Olson et al, a docking station may be provided with active cooling mechanisms such as heat pipes, heat slugs, cooling coils, and the like to transfer heat from the portable computer when docking is detected.
Considering, however, the superior cooling capabilities and low noise qualities of liquid-based cooling systems, it would be desirable in the art for a cooling system which would incorporate the advantages of liquid based cooling in a quiet environment for portable devices.